Without exception all of the hydraulic controls of the present art are based on the principle of throttling, which converts the energy of flow and pressure into heat.
In this specification a negative type load means an aiding type load, while a positive type load means a resistive type load that is a load which absorbs energy supplied from the pump to perform useful work.
During control of positive load, depending on the type of hydraulic system and especially on the type of pump used in the system, various amounts of throttling take place. The less the amount of throttling used in control of a positive load the more efficient the fluid power and control system.
The amount of throttling in control of positive type loads was greatly reduced by the introduction of load responsive, or load sensing systems, in which only a relatively small amount of throttling takes place such as, U.S. Pat. No. 3,470,694, issued Oct. 7, 1969 to T. Budzich.
Through application of the principle of variable pressure differential to such a load responsive system, the efficiency of such system can be still further increased by further reduction in throttling and the already high quality of the control of such a load responsive system and its valves can be further increased. Such a system is shown in U.S. Pat. No. 4,285,195, issue Aug. 25, 1981 to T. Budzich. However, those increases in efficiency and reduction in throttling of these load responsive systems only applies to the control of positive load, while the control of negative load, in all known compensated systems of the present art is done by totally converting, by the throttling process, the pressure-flow energy directly into heat. A typical system is shown in U.S. Pat. No. 3,744,517, issued July 10, 1973 to T. Budzich. Various negative load controls of the present art have this common feature. The only difference between those controls being the quality of the control, which for example was greatly improved through application of the concept of compensation.
Generation of heat, by throttling the fluid, not only represents a reduction in the efficiency of the system, but also introduces other parasitic effects, like for example an increase in temperature of the working fluid, necessitating the use of heat exchangers of various types and generally reducing the useful life of the fluid. Like, for example, the hydraulic oil and increasing the hazard to the operators of such systems.